1. Field of the Invention
The present invention relates to: a wavelength plate which applies different polarization conversions to light of different wavelengths; and an optical pickup having a laser light source for emitting light of different wavelengths, the optical pickup being used for recording a signal to or reproducing a signal from optical disks of different specifications or standards, e.g., CDs, DVDs, or Blu-ray discs.
2. Description of the Related Art
In recent years, optical disk apparatuses to be used in external storage apparatuses for computers or video recorders or the like might encounter optical disks of various standards (concerning recording density/capacity, groove specifications, substrate thickness, and the like), e.g., CDs (CD-ROM disks, CD-R disks, CDRW disks), DVDs (DVD-ROM disks, DVD-RAM disks, DVD-R disks, DVD-RW disks), or Blu-ray discs. There has been an increase in the number of “multi-disk purpose” apparatuses, each of which can support such a multitude of optical disks by itself.
Such an apparatus includes an optical pickup as an interface with which to write information (data) to or read data from an optical disk. This optical pickup incorporates a plurality of laser light sources of different wavelengths, and photodetectors for generating various signals (an RF signal, focusing and tracking control signals) in response to light reflected from an optical disk. A multi-disk apparatus selects a laser light source and an optical system in accordance with the type of the optical disk which has been loaded, and performs a data write, erase, or read for that optical disk.
Such a multi-disk apparatus is required to support optical disks of various standards and yet have a small size and low costs. To achieve this, it is important to reconcile versatility with respect to different types of optical disks and compactness of the optical system of the optical pickup. Known examples of optical pickups are described in Japanese Laid-Open Patent Publication No. 2000-132848, for example.
In order to attain versatility with respect to different types of optical disks, it is necessary to guarantee a stable signal reproduction performance for optical disks having thick transparent substrates and a large birefringence, for example. On the other hand, in order to keep the optical system compact, it is necessary that the optical elements be shared with respect to light of different wavelengths, thus reducing the number of elements.
Hereinafter, with reference to FIG. 14 and FIGS. 15A to 15C, the structure and operation of an optical pickup which the inventors have disclosed in Japanese Patent Application No. 2005-121245 (hereinafter referred to as the “prior JP application”) will be described. Note that this prior JP application is not yet published as of the filing date of the present application.
The optical pickup apparatus shown in FIG. 14 comprises: a light source 101 for generating a plurality of light beams; a collimating lens 104 for collimating a light beam; a polarization element 107, composed of a hologram 105 and a wavelength plate 106; an objective lens 108 for converging a light beam and forming a light spot on a signal surface 109 or 110 of an optical disk; and a photodetector 103 for detecting the intensity of a light beam which has been reflected from the signal surface 109 or 110 of the optical disk. Together with the objective lens 108, the polarization element 107 is attached to a supporting member 135, and thus is integrally driven with the objective lens 108 by an actuator 136.
The photodetector 103 is formed on a semiconductor substrate 102 such as a silicon chip. The substrate 102 has the light source 101 mounted thereon, which is composed of laser chips for emitting two types of laser light, i.e., a length λ1 and a wavelength λ2. The wavelength λ1 is about 650 nm, and the wavelength λ2 is about 800 nm. Laser light of the wavelength λ1 is used for DVDs, whereas laser light of the wavelength λ2 is used for CDs. The photodetector 103 is composed of a plurality of photodiodes for converting light into electrical signals via photoelectric effects. It is assumed that an optical disk having the signal surface 109 is a DVD, whereas an optical disk having the signal surface 110 is a CD. Although FIG. 14 illustrates two optical disks at the same time, one of the optical disks is to be loaded to the apparatus in actuality.
Light of the wavelength λ1 which has been radiated from the light source 101 is collimated by the collimating lens 104, and thereafter transmitted through the polarization element 107. The polarization element 107 is an integrated optical element composed of the polarization-type hologram 105 and the wavelength plate 106. The light (wavelength λ1) which has been transmitted through the polarization element 107 is converged onto the recording surface 109 of the optical disk (DVD) by the objective lens 108, and reflected therefrom. The reflected light passes back through the objective lens 108 so as to enter the polarization element 107. Due to the polarization dependence of the polarization element 107, the reflected light is diffracted by the polarization element 107.
A portion of the light which has been diffracted by the polarization element 107 passes through the collimating lens 104 so as to enter the photodetector 103. The photodetector 103 generates electrical signals which are in accordance with changes in the light amount (a focusing control signal, a tracking control signal, and an RF signal).
When a CD is loaded instead of a DVD, light of the wavelength λ2 is radiated from the light source 101. The light of the wavelength λ2 which has been radiated from the light source 101 is also collimated by the collimating lens 104, and transmitted through the polarization element 107. The light transmitted through the polarization element 107 is converged onto the recording surface 110 of the optical disk by the objective lens 108, and reflected at the recording surface 110. The reflected light passes back through the objective lens 108 and is diffracted by the polarization element 107. The diffracted light passes through the collimating lens 104 so as to enter the photodetector 103. The photodetector 103 generates electrical signals which are in accordance with changes in the light amount.
Thus, the above-described optical pickup includes a single light source 101 which radiates light of two different wavelengths, i.e., one for DVDs and another for CDs, as well as the common photodetector 103 which receives light of different wavelengths that is reflected from an optical disk.
In accordance with this structure, there is realized a compact optical pickup which supports optical storage media of different standards. The reason is that there is no need to use any splitting means for splitting light of different wavelengths along the optical path, and that an optical path (forward path) from the light source 101 to the optical storage medium and an optical path (return path) from the optical storage medium to the photodetector 103 can be commonly utilized for light of different wavelengths. As a result, the number of optical elements can be reduced, and the optical pickup can be made small.
FIG. 15A shows a plan view of the wavelength plate 106 according to the prior JP application as illustrated in FIG. 14. FIG. 15B is a diagram illustrating how the light traveling from the light source toward the optical disk 110 and the reflected light from the optical disk 110 are led forth and back through the wavelength plate 106. FIG. 15C is a diagram illustrating exemplary polarization conversion.
FIG. 15A shows the planar structure of the wavelength plate 106. The wavelength plate 106 is divided into four regions by lines (x axis, y axis) extending through the optical axis center. The four regions are: two regions A each having an optic axis which constitutes an angle of θ1 with respect to the x axis direction; and two regions B each having an optic axis which constitutes an angle of θ2 with respect to the x axis direction. Thus, the wavelength plate 106 has a plurality of birefringent regions whose optic axes extend in different directions, such that regions having the same property are present at 180° rotation symmetrical positions with respect to the optical axis extending through the origin of x-y coordinates.
It is assumed that, when light (linearly polarized light) which is radiated from the light source enters the wavelength plate 106, the polarization direction (i.e., the direction in which the electric field vector vibrates) of the light is parallel to the x axis. The angles θ1 and θ2 are, for example, 45°−α and 45°+α (where 0<α≦15°) with respect to the x axis direction, respectively. Any wavelength plate having a distribution of such regions of different properties will hereinafter be referred to as a “distributed-type wavelength plate”.
Among the rays which are radiated from the light source and enter the wavelength plate 106, those rays which pass through a region A are converged onto the optical disk 110 by the lens 108, and reflected from the optical disk 110. After being transmitted back through the lens 108, the reflected light will pass through the other region A, which is at a symmetric position with respect to the optical axis. Similarly, those rays which pass through a region B will travel through the other region B after being reflected from the optical disk 110.
Assuming that the wavelength plate 106 has a refractive index anisotropy of Δn and a thickness of d, and that the wavelength of laser light for DVDs is λ1, a retardation of the wavelength plate 106 (which is expressed as 360°×Δnd/λ) is prescribed to be 90°. If the value of α (which defines the direction of the optic axis) is zero, then the wavelength plate 106 functions as a conventional ¼ wavelength plate. If linearly polarized light (P-polarized light) having an electric field vector which is parallel to the x axis direction enters the ¼ wavelength plate, the light is converted into circularly polarized light, and is emitted from the wavelength plate. After being reflected from the optical information medium, the circularly polarized light travels through the wavelength plate, whereby it is converted into linearly polarized light (S-polarized light component) having an electric field vector which is parallel to the y axis direction.
Since α is not zero in the wavelength plate 106 shown in FIG. 15A, different polarization conversions are applied to light which is transmitted back and forth through the regions A and to the light which is transmitted back and forth through the regions B. However, since α is small (0<α≦15°), there is a smaller difference in polarization state than in the case where light is transmitted back and forth through a conventional ¼ wavelength plate. Therefore, the return path light having the wavelength λ1 enters the hologram 105 in a polarization state similar to the polarization state in the case where the light is transmitted through a ¼ wavelength plate having a substantially uniform optic axis direction.
On the other hand, with respect to laser light for CDs (wavelength λ2), the retardation of the wavelength plate 106 (which is generally in inverse proportion with the wavelength) is about 75° (=90°×650/800≈90°×5/6). Therefore, when P-polarized light enters the wavelength plate 106, the light becomes elliptically polarized light, and is emitted from the wavelength plate as such. When the light reflected from the optical information medium 110 travels back through the wavelength plate 106, the light is converted into elliptically polarized light, with the direction of the major axis of the ellipse being changed. The major axis of the ellipse is substantially parallel to the y axis direction, so that there is a higher proportion of the S-polarized light component. The polarization state of laser light for CDs having the wavelength λ2 changes depending on whether it is transmitted through the regions A or the regions B, and the difference therebetween is greater than that for light of the wavelength λ1.
FIG. 15C shows changes in the polarization state of laser light for CDs (wavelength λ2). As described above, when linearly polarized light I having an electric field vector which is parallel to the x axis direction is transmitted through the wavelength plate 106, the light is subjected to different polarization conversions depending on whether the light is transmitted through a region A or a region B. The linearly polarized light I is converted to elliptically polarized light II by the wavelength plate 106. If the optical disk 110 has no birefringence, light which is reflected from the optical disk 110 and travels back through the wavelength plate 106 is converted into elliptically polarized light III, the major axis of whose ellipse is in a direction as shown in FIG. 15C.
1 On the other hand, in the case where the optical disk 110 has birefringence, as shown in FIG. 15C for example, the light which is transmitted through the regions A is converted into light III′, which has the same polarization state as that of the linearly polarized light I. The light III′ is not diffracted by the polarization hologram 105 of FIG. 14, but returns to the light source 101. As a result, the photodetector 103 cannot detect the light III′.
On the other hand, light which is transmitted through the regions B is subjected to a different polarization conversion from that which is applied to the light transmitted through the regions A. As shown in FIG. 15C, even if the optical disk 110 has birefringence, the light III′ becomes elliptically polarized light having an S-polarized component, and therefore is diffracted by the polarization hologram 105 of FIG. 14.
Thus, with the wavelength plate 106 as shown in FIG. 15A, regardless of the amount of birefringence of the optical disk 110, there will exist a component of light which is transmitted through at least either the regions A or the regions B and diffracted by the polarization hologram. Therefore, there will always be some diffracted light entering the photodetector 103.
Even in the case where the optical disk has no birefringence, the signal light amount ascribable to light of the wavelength λ2 is smaller than the signal light amount ascribable to light of the wavelength λ1. This is because light of the wavelength λ2 does not satisfy the perfect diffraction condition in its return path. Although a CD has a thick substrate and therefore is likely to acquire a large birefringence during the fabrication process, it is relatively easy to produce high power laser light for CDs; therefore, rather than trying to efficiently direct reflected light into the photodetector, it would be more preferable to employ a distributed-type wavelength plate for securement of signal light. On the other hand, a DVD has a thin substrate thickness and therefore its substrate is unlikely to acquire birefringence during the fabrication process; however, laser light for DVDs tends to have a short wavelength and low power, and therefore reflected light must be directed into the photodetector with a high efficiency.
Among the optical disks that are actually on the market which have thick substrates, e.g., CDs, some may have extreme birefringence fluctuations. In worst cases, the substrate of an optical disk may function as a ½ wavelength plate. In such cases, light which passes through the wavelength plate and enters the polarization hologram will not be diffracted by the polarization hologram. In the case where the substrate of an optical disk has such an extremely large birefringence, the problematic decrease in the signal light amount will not be adequately overcome even by employing the wavelength plate 106 as shown in FIG. 15A.
FIG. 16 is a graph illustrating the relationship between degrees of birefringence (phase difference) of an optical disk and signal characteristics, where the aforementioned wavelength plate 106 is employed. In FIG. 16, (a) relates to CDs, whereas (b) relates to DVDs. The retardation of the wavelength plate 106 used is 90° with respect to laser wavelength for DVDs, and 75° with respect to the laser light wavelength for CDs. The optic axis directions of the regions A and the regions B are 45°±10°, respectively, with respect to the x axis. The horizontal axis of the graph represents the phase difference which is imparted to the light traveling back and forth through the optical disk substrate, whereas the vertical axis represents the jitter value (Jitter), DC level, and AC amplitude of the reproduction signal.
FIG. 17 is a graph illustrating the relationship between degrees of birefringence (phase difference) of an optical disk and signal characteristics, where a conventional wavelength plate having a uniform optic axis direction is employed. In FIG. 17, (a) relates to CDs, whereas (b) relates to DVDs. The retardation of the wavelength plate 106 used is 90° with respect to laser wavelength for DVDs, and 75° with respect to the laser light wavelength for CDs. The optic axis direction of the wavelength plate is 45° with respect to the x axis. The horizontal axis of the graph represents the phase difference which is imparted to the light traveling back and forth through the optical disk substrate, whereas the vertical axis represents the jitter value (Jitter), DC level, and AC amplitude of the reproduction signal (RF signal).
In the case where a conventional uniform wavelength plate is employed, as seen from FIG. 17, the DC level and AC amplitude of the reproduction signal become zero when the optical disk has a 90° birefringence. On the other hand, in the case where a distributed-type wavelength plate is employed, as seen from FIG. 16, neither the DC level nor the AC amplitude of the reproduction signal would become zero. However, its signal level is reduced to 1/10 or less of the signal level in the case where there is a 0° birefringence, thus indicating an extremely low signal-to-noise ratio (S/N), with an outstanding decrease in AC amplitude. This is presumably because the spatial frequency characteristics are deteriorated due to non-uniformity within the cross section of rays which are diffracted by the hologram in the return path.
In the case where a uniform wavelength plate is employed, the jitter value becomes infinite when the birefringence is 75° or more, as can be seen from FIG. 17. In the case where a distributed-type wavelength plate is employed, as seen from FIG. 16, the jitter value does not become infinite even when the birefringence is 90° , but is still as large as about 15%, thus indicating a serious deterioration in the reproduction signal.
Although the same is also true of laser light for DVDs, since a DVD has a thin substrate, the amount of birefringence which would occur during the fabrication process is ±60° or less. Therefore, the jitter aggravation is within a tolerable range.
Thus, even by employing a distributed-type wavelength plate as shown in FIG. 15A, only a reproduction signal with a deteriorated quality can be obtained from an optical disk which has an extremely large amount of birefringence. One way to solve the problem of degradation of the signal light might be to employ circuitry for amplifying the signal. However, since the birefringence of the optical disk greatly varies between e.g. the inner periphery and the outer periphery of a single optical disk, frequent switching of the signal amplification gain would be required for a single optical disk, which is not practical.
It might seem effective to increase the separation angle a between the optic axis directions of the respective regions of the distributed-type wavelength plate, in order to increase the amount of light which enters the detector when the amount of birefringence is 90°. However, increasing the separation angle α would result in an interference between the rays transmitted through the respective regions of the wavelength plate, and hence deterioration in signal quality.
Moreover, the number of divided regions having different optic axes is preferably greater than the number illustrated in FIG. 15A. By increasing the number of divided regions, signal characteristics with lower jitter values are realized. The reason why the jitter value increases as the birefringence of the disk substrate increases as shown by the graph of (a) of FIG. 16 is that the deviations in optical intensity across the ray cross section of the return path light increase as the birefringence increases, thus resulting in a poor convergence quality of the light spot that is converged on the detection surface.